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BIO2305
Fluid, Electrolyte, & Acid-Base Balance
Fluid, Electrolyte, and Acid-Base Balance
Fluid balance
The amount of water gained each day equals the amount lost
Electrolyte balance
Ion gain per day = ion loss per day
Acid-base balance
H+ gain is offset by their loss
Body Water Content
In the average adult, body fluids comprise about 60% of total body
weight
Body fluids occupy two main compartments:
Intracellular fluid (ICF) – about two thirds by volume,
cytosol of cells
Extracellular fluid (ECF) – consists of two major
subdivisions
Plasma – the fluid portion of the blood
Interstitial fluid (ISF) – fluid in spaces between cells
Other ECF – lymph, cerebrospinal fluid, eye humors,
synovial fluid, serous fluid, and gastrointestinal
secretions
Composition of Body Fluids
Water is the main component of all body fluids - making up 45% to 75% of the total body weight
Solutes are broadly classified into:
Electrolytes – inorganic salts, all acids and bases, and some proteins
Nonelectrolytes – examples include glucose, lipids, creatinine, and urea
Electrolytes have greater osmotic power than nonelectrolytes
Water moves according to osmotic gradients
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Electrolyte Composition of Body Fluids
Each fluid compartment of the body has a fairly distinctive pattern of electrolytes
Interstitial fluids are similar to blood (except for the high protein content of plasma)
Sodium is the chief cation
Chloride is the major anion
Intracellular fluids have low sodium and chloride
Potassium is the chief cation
Phosphate is the chief anion
Extracellular and Intracellular Fluids
[Na+] and [K+] in ECF and ICF are nearly opposite
High [Na+] in ECF
High [K+] in ISF
Reflects the activity of cellular ATP-dependent
Na+/K+ Pump
Electrolytes determine the chemical and physical
reactions of fluids
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Regulation Of Fluids And Electrolytes
Homeostatic mechanisms respond to changes in ECF
Respond to changes in plasma volume or osmotic concentrations
Water movement between ECF and ICF moves passively in response to osmotic gradients
Water Balance and ECF Osmolality
To remain properly hydrated, water intake must equal water output
Water intake sources
Ingested fluid (60%) and solid food (30%)
Metabolic water or water of oxidation (10%)
Water output
Urine (60%) and feces (4%)
Insensible losses (28%), sweat (8%)
Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH)
Water Intake and Output
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Regulation of Water Intake
The hypothalamic thirst center is stimulated:
decline in plasma volume of 10%–15%
increases in plasma osmolality of 1–2%
Via baroreceptor input, angiotensin II, and other stimuli
Feedback signals that inhibit the thirst centers include:
Moistening of the mucosa of the mouth and throat
Activation of stomach and intestinal stretch receptors
Regulation of Water Output
Obligatory water losses include:
Insensible water losses from lungs and skin
Water that accompanies undigested food residues in feces
Obligatory water loss reflects the fact that:
Kidneys excrete 900–1200 mOsm of solutes to maintain blood homeostasis
Urine solutes must be flushed out of the body in water
Primary Regulatory Hormones
Antidiuretic hormone (ADH)
aka Vasopressin
Stimulates water conservation and the thirst center
Aldosterone
Controls Na+ absorption and K+ loss along the DCT
Natriuretic peptides (ANP and BNP)
Reduce thirst and block the release of ADH and aldosterone
Electrolyte Balance
Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance
Salts are important for:
Neuromuscular excitability
Secretory activity
Membrane permeability
Controlling fluid movements
Sodium in Fluid and Electrolyte Balance
Sodium holds a central position in fluid and electrolyte balance
Sodium is the single most abundant cation in the ECF
Accounts for 90-95% of all solutes in the ECF
Contribute 280 mOsm of the total 300 mOsm ECF solute concentration
The role of sodium in controlling ECF volume and water distribution in the body is a result of:
Sodium being the only cation to exert significant osmotic pressure
Sodium ions leaking into cells and being pumped out against their electrochemical gradient
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Sodium Balance
Sodium concentration in the ECF normally remains stable
Rate of sodium uptake across digestive tract directly
proportional to dietary intake
Sodium losses occur through urine and perspiration
Changes in plasma sodium levels affect:
Plasma volume, blood pressure
ICF and interstitial fluid volumes
Large variations corrected by homeostatic mechanisms
Too low: ADH & aldosterone secreted
Too high: ANP secreted
Sodium Balance
Sodium concentration in the ECF normally remains stable
Rate of sodium uptake across digestive tract directly
proportional to dietary intake
Sodium losses occur through urine and perspiration
Changes in plasma sodium levels affect:
Plasma volume, blood pressure
ICF and interstitial fluid volumes
Large variations corrected by homeostatic mechanisms
Too low, ADH / aldosterone secreted
Too high, ANP secreted
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Regulation of Sodium Balance with ANP
Atrial Natriuretic Peptide (ANP)
Promotes excretion of sodium and water
Inhibits angiotensin II production
Deactivates Na+/Cl- symport cotransporter (NCC) to prevent Na+ reabsorption from filtrate in
DCT
Uses cGMP/2nd messenger system to deactivate ENaC’s (Epithelial sodium channels) in the
Collecting Duct of the nephron
Regulation of Sodium Balance with Aldosterone
The Renin-Angiotensin mechanism triggers the release of Aldosterone
Sodium reabsorption:
65% of sodium in filtrate is reabsorbed in the proximal tubules
25% is reclaimed in the loops of Henle
When aldosterone levels are high, all remaining Na+ is actively reabsorbed
Water follows sodium ONLY IF tubule permeability has been increased with ADH
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Potassium Balance
K+ concentrations in ECF are normally very low
Not as closely regulated as sodium
K+ excretion increases as ECF concentrations rise due to the release of aldosterone
K+ retention increases when pH falls (H+ secreted in exchange for reabsorption of K+ in DCT)
Regulation of Potassium Balance
Relative ICF-ECF K+ ion concentration affects a cell’s resting membrane potential
Excessive K+ in ECF decreases membrane potential
Too little K+ in ECF causes hyperpolarization and unresponsiveness
+
K controls its own ECF concentration via feedback regulation of aldosterone release
Increased K+ in the ECF around the adrenal cortex causes release of aldosterone
Aldosterone stimulates K+ ion secretion via Na+ retention
In cortical collecting ducts, for each Na+ reabsorbed, a K+ is secreted
Na+ and K+ generally travel in opposite directions across membranes
When K+ levels are low, the amount of K+ secretion and excretion in urine is kept to a
minimum
Regulation of Calcium
Ionic calcium (Ca2+) in ECF is important for:
Blood clotting (Intrinsic, Extrinsic, and Common Coagulation Pathways)
Cell membrane permeability
Secretory behavior
Calcium balance is controlled by Parathyroid Hormone (PTH) and Calcitonin
Low Ca2+ levels stimulates release of PTH which stimulates:
Osteoclasts to break down bone matrix
Intestinal absorption of calcium
High Ca2+ levels stimulate thyroid to produce calcitonin which stimulates
Ca2+ secretion in kidneys
Ca2+ deposition in bone
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Regulation of Calcium
Regulation of Anions
Chloride (Cl-) is the major anion accompanying Na+ in the ECF
99% of Cl- is reabsorbed under normal pH conditions
Na+ concentration gradient operates Na+/Cl- symporter in DCT
When acidosis occurs, fewer chloride ions are reabsorbed
Other anions have transport maximums and excesses are excreted in urine
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Acid-Base Balance
Normal pH of body fluids
Arterial blood is 7.4
Venous blood and interstitial fluid is 7.35
Intracellular fluid is 7.0
Important part of homeostasis because cellular
metabolism depends on enzymes
Enzymes are proteins and are sensitive to pH
Challenges to acid-base balance due to cellular
metabolism: produces acids (hydrogen ion donors)
Sources of Hydrogen Ions
Most H+ originate from cellular metabolism
Breakdown of phosphorus-containing proteins
releases phosphoric acid into the ECF
Anaerobic respiration of glucose produces lactic
acid
Fat metabolism yields organic acids and ketone
bodies
Transporting carbon dioxide as bicarbonate
releases hydrogen ions
Hydrogen Ion Regulation
Concentration of hydrogen ions is regulated sequentially by:
Chemical buffer systems – act within seconds
Respiratory center in the brain stem – acts within 1–3 minutes
Renal mechanisms – require hours to days to effect pH changes
Chemical Buffer Systems
Strong acids:
100% of molecules dissociate in water into H+ and anions
H+ ions released rapidly
Weak acids:
≤ 5% of molecules dissociate in water into H+ and anions
Weak acids are efficient at preventing pH changes
Strong bases:
100% of molecules dissociate in water into OH- and cations
Quickly tie up H+
Weak bases:
≤ 5% of molecules dissociate in water into OH- and cations
Accept H+ more slowly
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Chemical Buffer Systems
One or two molecules that act to resist pH changes when strong acid or base is added
Three major chemical buffer systems
Bicarbonate buffer system
Phosphate buffer system
Protein buffer system
Any deviations in pH are resisted by the entire chemical buffering system
Bicarbonate Buffer System
A mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3) (potassium or
magnesium bicarbonates work as well)
If strong acid is added:
Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak
acid)
The pH of the solution decreases only slightly
If strong base is added:
It reacts with the carbonic acid to form sodium bicarbonate (a weak base)
The pH of the solution rises only slightly
Bicarbonate system is the most important ECF buffer
Phosphate Buffer System
This system is an effective buffer in urine and intracellular fluid (ICF)
Works much like the Bicarbonate system
System involves:
Sodium dihydrogen phosphate (NaH2PO4-)
OH- + H2PO4-  H2O + HPO42Sodium monohydrogen phosphate (Na2HPO42-)
H+ + HPO42-  H2PO4-
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Protein Buffer System
Plasma and intracellular proteins are the body’s most plentiful and powerful buffers
Some amino acids of proteins have:
Free organic acid groups (weak acids)
Groups that act as weak bases (e.g., amino groups)
Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base
Protein Buffer System
Plasma and intracellular proteins are the body’s most plentiful and powerful buffers
Some amino acids of proteins have:
Free organic acid groups (weak acids)
Groups that act as weak bases (e.g., amino groups)
Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base
Respiratory Mechanism of Acid-Base Balance
The respiratory system regulation of acid-base balance is a physiological buffering system
When hypercapnia or rising plasma H+ occurs:
Deeper and more rapid breathing expels more CO2
[H+] is reduced
Alkalosis causes slower, more shallow breathing, causing H+ levels to increase
Respiratory system impairment causes acid-base imbalance (respiratory acidosis or respiratory
alkalosis)
Respiratory Acidosis and Alkalosis
Result from failure of the respiratory system to balance pH
pCO2 is the single most important indicator of respiratory inadequacy
Blood pCO2 levels:
Normal pCO2 fluctuates between 35 and 45 mm Hg
Values above 45 mm Hg signal respiratory acidosis
Values below 35 mm Hg indicate respiratory alkalosis
Respiratory Acidosis and Alkalosis
Respiratory acidosis is a common result of hypoventilation
The most common cause of acid-base imbalance
Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as
pneumonia, cystic fibrosis, or emphysema
Respiratory alkalosis is a common result of hyperventilation
Breathing deeply and rapidly, lowering pCO2 levels to <35 mm Hg
Defined as a rate of Pulmonary Ventilation that is greater than that needed to maintain a
homeostatic pCO2
Breathing heavy during exercise is not hyperventilation because it is necessary for
pCO2 homeostasis
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Respiratory Acidosis and Alkalosis
Renal Mechanisms of Acid-Base Balance
Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body
The lungs can eliminate carbonic acid by eliminating carbon dioxide
Only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids and ketones)
and prevent metabolic acidosis
The ultimate acid-base regulatory organs are the kidneys
Renal Mechanisms of Acid-Base Balance
The most important renal mechanisms for regulating acid-base balance are:
Conserving (reabsorbing) or generating new bicarbonate ions
Excreting bicarbonate ions
Losing a HCO3- ion is the same as gaining a H+ ion
Reabsorbing a HCO3- ion is the same as losing a H+ ion
Intercalated Cells in PCT move H+
Intercalated = interposed
“inter-” means between
“-calatum” means inserted
Two types of intercalated cells:
Type A intercalated cells
H+/ATPase on Apical Side
Secrete protons
Cl /HCO3- cotransporter on Basolateral side
Reabsorb bicarbonate
Type B intercalated cells
H+/ATPase on Basolateral Side
Reabsorb protons
Cl-/HCO3- cotransporter on Apical side
Secret bicarbonate
Bicarbonate Reabsorption / H+ Excretion
In response to acidosis, kidneys generate bicarbonate ions and
add them to the blood
An equal amount of H+ ions are added to the urine
H+ ions must bind to buffers in the urine when excreted
For each H+ ion excreted, a Na+ ion and a bicarbonate (HCO3-)
ion are reabsorbed by the PCT cells
Bicarbonate Excretion / H+ Reabsorption
When the body is in alkalosis, tubular cells:
Secrete bicarbonate ions
Reclaim hydrogen ions and acidify the blood
The mechanism is the opposite of bicarbonate ion reabsorption process
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Metabolic pH Imbalance
Metabolic acidosis is the second most common cause of acid-base imbalance
Typical causes are:
Ingestion of too much alcohol and excessive loss of bicarbonate ions
Other causes include accumulation of lactic acid, shock, ketosis in diabetic crisis,
starvation, and kidney failure
Metabolic alkalosis due to a rise in blood pH and bicarbonate levels
Typical causes are:
Vomiting of the acid contents of the stomach
Intake of excess base (e.g., from antacids)
Constipation, in which excessive bicarbonate is reabsorbed
Respiratory and Renal Compensations
Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the
other system
The respiratory system will attempt to correct metabolic acid-base imbalances
The kidneys will work to correct imbalances caused by respiratory disease
Response to Metabolic Acidosis
Pulmonary ventilation (rate and depth of breathing) is elevated
As carbon dioxide is eliminated by the respiratory system, pCO2 falls below normal
Kidneys secrete H+ and retain/generate bicarbonate HCO3- to offset the acidosis
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Response to Metabolic Alkalosis
Pulmonary ventilation is slow and shallow allowing carbon dioxide to accumulate in the blood
Kidneys generate H+ and eliminate HCO3- from the body by secretion
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